net: tests: Tweak the IP address test to use new net_if API

Instead of using hard coded net_if, get the default network
interface using net_if_get_default() function.

Change-Id: I6248555ce57ae5196adea7baa9db325660a71255
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>